Resonant mechanical scanners are employed in a wide variety of applications. Resonant mechanical scanners are widely used because of their high reliability, long life and low power consumption. Sinusoidally oscillated mechanical scanners have minimum energization requirements and are mechanically and electronically simple in design, fabrication and operation. A sinusoidally driven scanning system, however, is inherently limited in that the scanning velocity varies throughout the entire scan angle. A sinusoidal drive signal produces a scanning velocity which is a time varying value, i.e., non-constant, especially adjacent the sinusoidal maxima and minima.
Thus, a sinusoidal scanning mirror experiences non-linear scanning due to a reduction in scan velocity as the sinusoidal maxima and minima are approached. Non-linear scanning reduces the overall scan efficiency of the system.
Non-linear scan velocities due to sinusoidal drive signals may be compensated for by a variety of mechanical, optical and/or electronic techniques. One possible mechanical technique is overscanning wherein the amplitude of the sinusoidal drive signal is increased so that the mechanical scanner is driven through a wider-than-necessary scan angle. Thus, in effect only the linear velocity portion of the sinusoidal drive signal is utilized. Since a portion of the scan angle is not being utilized by the scanning system, however, such a mechanical technique results in very poor scan efficiency.
Another mechanical technique for improving scan linearity is to combine a number of resonant mechanical scanners having appropriate amplitudes, phase relations and frequencies (a predetermined fundamental frequency and one or more exact harmonics thereof) to form an optical scan pattern which is the result of the superposition of the beam deflection of each of the individual resonant mechanical scanners. Such a system is relatively complex both mechanically and electronically.
Another means of obtaining scan linearity involves the use of an optical element in conjunction with the resonant mechanical scanner. An optical element disposed in front of the mechanical scanner causes a divergence in the scanning beam at the end of the scan angle, the outward divergence of the beam by the optical element compensating for the reduction in scan velocity due to the sinusoidal drive signal. The optical element in such a system, however, must be designed and fabricated to precisely match the particular scan amplitude of the resonant mechanical scanner, a decided limitation. Further, using an optical element to diverge the scan also increases the beam width.
One electronic technique to compensate for the non-linear scan generated by a sinusoidal drive signal utilizes electronic circuitry to subdivide the period of the sinusoidal drive signal into a number of equal subparts. Those subparts of the period adjacent the sinusoidal maxima/minima are not used for scanning operations. This technique is somewhat similar to the mechanical overscanning technique in that the scanning system is non-operational during a portion of the sinusoidal drive signal, thereby resulting in very poor scan efficiency.
To overcome the inherent limitations of resonant mechanical scanners of the prior art as described in the preceding paragraphs, a dual-mode resonant scanning system as described and claimed in U.S. Pat. No. 4,859,846 was conceived and implemented. The dual-mode resonant scanning system of the '846 patent is directed to a dual-mode resonant mechanical scanner wherein the resonant mechanical scanner is simultaneously oscillated at a predetermined fundamental frequency and an exact third harmonic thereof to provide constant velocity scanning through a predetermined scan angle.
The dual-mode resonant scanning system described in the '846 patent is a high-Q system wherein Q is a figure of merit which reflects the energy losses in a resonant system, a high-Q system having minimal energy losses. The scanning system of the '846 patent comprises a high-Q dual-mode resonant mechanical scanner and an electronic control circuit which generates the exact third harmonic of the predetermined fundamental frequency and phase locks the exact third harmonic oscillation to the predetermined fundamental frequency oscillation.
As described in the '846 patent with reference to FIG. 4, the dual-mode resonant scanning system of the '846 patent generates an output signal having a sharp response curve 70 at the predetermined fundamental frequency and an output signal having a sharp response curve 72 at the exact third harmonic of the fundamental frequency by means of a multiplier circuit 42. These output signals are additively combined in a summing circuit 46 to generate a drive signal which produces a scan motion 29 as illustrated in FIG. 3B.
The inventors have observed that in the dual-mode resonant scanning system described in the '846 patent the "Q" of the exact third harmonic oscillation is generally higher than the "Q" of the predetermined fundamental frequency. That is, the response curve 72 of the exact third harmonic is sharper than the response curve 70 of the predetermined fundamental frequency even though the fundamental oscillates with an amplitude of about some ten times greater than the amplitude of the harmonic oscillation. It is postulated that losses due to air loading on the mirror surface may be much greater for the fundamental oscillation than for the exact third harmonic oscillation which causes the observed variations in "Q" of the response curves.